It is known to form relatively large blocks or "buns" of polyurethane foam, from which many types of useful articles can be constructed. For example, the foam bun can be sliced into smaller pieces, from which can be made seat cushions, mattresses and the like.
Production of these polyurethane foam buns involves the pouring of the foam mixture when in a liquid state from a pour trough onto a fall plate. As the mixture reacts, the foam transitions from a liquid state to a "creaming" state by virtue of a blowing agent which generates thermal expansion of the foam. During this creaming phase, the mixture rises and then subsequently hardens, or sets to form cellular solid foam. The mixture advances along the fall plate and onto and along a mechanically operated endless band conveyor. The block of foam, or bun, is then removed from the conveyor, and is allowed to "set" or "cure" for the proper amount of time to allow the mixture to form the solid cellular foam.
A disadvantage in the conventional production of polymeric foam buns is the amount of time required for a typical foam bun (which would be on the order of 8 feet wide.times.60 feet long.times.40 inches tall) to sufficiently cure such that products can be fabricated from the bun. Typically this has required on the order of 48 hours. This is because the interior of the bun can remain at a very high temperature (around 300.degree. F.) for a number of hours, due to the mass of such buns and the excellent heat insulating qualities of the polyurethane foam.
Another disadvantage with conventional foam bun production techniques is that, due to the aforementioned mass and heat insulating qualities of the polyurethane foam buns, the interiors of the bun in cooling at a slower rate than the exterior surfaces of the bun create, when cured, a bun which has mechanical properties which can vary greatly through the cross section of the bun. Specifically, the buns are generally harder in the center and softer on the vertical sides and upper and lower surfaces of the bun.
The ASTM 3574-D test standard for foam contains the Indention Force Deflection ("I.F.D.") test. This test standard measures the amount of force required to press an 8 inch diameter metal disc into the top surface of a test specimen a distance equal to 25% of the thickness of the test specimen. The foam industry has accepted 4 inches as the standard thickness, so the I.F.D. test would be based on a 1 inch indentation. The round disc is outfitted with a load cell or scale to record the force required. Foams are fabricated for multiple uses, and the force from the I.F.D. test can be anywhere from less than one lb. to 100 lbs., depending on the grade of foam and its particular application. A standard value for foam employed in couch cushions, chair cushions and the like would be foam having an I.F.D. on the order of 25-30 lbs. With conventional foam bun production techniques, the I.F.D. spread on 25-30 lb. foam can be as much as 7 lbs. on low density foam and as high as 12 lbs. on high density foam. A spread of that magnitude is particularly undesirable because when couch cushions (typically three for a standard couch) are cut from the same bun, the cushions are noticeably of a different firmness, which can readily be detected when moving from seated atop one cushion to being seated atop another cushion of the couch.
One solution to shortening the curing period for polyurethane foam buns and in the process creating more uniform cross sectional properties of the bun is disclosed in U.S. Pat. No. 3,890,414 to Ricciardi et al for a Rapid Cooling Process For Improving Quality Of Polyurethane Foam. Ricciardi discloses a process for improving the uniformity of the cross-sectional properties of a polyurethane foam bun comprising rapidly and uniformly cooling a bun of hot freshly polymerized foam by passing a large quantity of a cooling gas through the foam mass. Ricciardi discloses that preferably the gas is air that is drawn through the foam by applying a suction to one surface thereof.
However, such a technique as disclosed in Ricciardi has not met with complete success in remedying the aforementioned problems, namely the long cure times and non-uniform cross-sectional properties. Specifically, for the Ricciardi process to be successful, the opposed surfaces of the foam bun through which the cooling gas is drawn or forced, must be sufficiently porous to allow the cooling gas to easily pass through those surfaces and the bun itself. However, in the typical production of polymeric foam buns, a thick skin tends to form on the upper and lower surfaces of the bun during its production. A thick skin forms on the upper surface of the bun due to the fact that air comes into contact with the upper surface causing rapid cooling and loss of blowing agent and consequently the production of a skin thereon. A similar skin forms on the lower surface of the bun as the mixture flows downwardly over and along the fall plate on its way to reaching the conveyor. This is because the heat generated from the reacting mixture, which is traveling over the fall plate, is conducted into the fall plate, which is typically fabricated of steel. The fall plate can reach temperatures as high as 150.degree.-160.degree. F. This heat transfer from reacting mixture to fall plate therefore reduces the temperature of the foam mass at its lower surface and thereby reduces the amount of thermal expansion able to be generated by the blowing agent. This skin is somewhat thinner than the upper skin because of the heat returned to the foaming mixture from the fall plate after the fall plate has reached a temperature of 150.degree.-160.degree. F. This effect of the fall plate upon the foam tends to yield thinner, lighter skins in much the same way that mold temperature controls skin thickness in "internal skin" molding.
To remedy this problem and to allow the Ricciardi technique to work efficiently, Ricciardi teaches removing or puncturing this skin, which is generally densified and substantially nonporous, to allow the cooling gas to flow easily through. Ricciardi states that it is often necessary to completely remove a thin layer, from about 1/8 inch up to about 1 inch, from a pair of opposing surfaces of the foam block to expose a porous surface which will permit the passage of the cooling gas into, through and out of the foam block.
It will be appreciated, however, that such puncturing of the thickened skin, or entirely removing the thickened skin from the foam bun, is laborious and time-consuming. However, this step is required to enable air to be drawn through the bun. Having to perform this step tends to negate the efficiencies of rapidly cooling the foam bun by passing the cooling gas therethrough.
It has therefore been an objective of the present invention to provide an improved method of forming foam buns.
It has been another objective of the present invention to eliminate the need of puncturing or removing the thickened skin of opposing bun surfaces in order that a cooling gas can effectively be passed through the bun to cool and rapidly cure the bun.
It has been yet another objective of the present invention to prevent the forming of the thickened skin on the upper and lower surfaces of a foam bun during production thereof, such that porous upper and lower surfaces will be immediately exposed so that the foam bun can be rapidly cooled by passing a cooling gas through the bun without any other preparatory steps being required.